1. Field of the Invention
The invention relates generally to a heating element for heating flowing media and, more particularly, to a heating element including a heat exchanger having a metallic body composed of aluminum, copper or alloys containing a high proportion of aluminum or copper and which has regularly arranged passageways extending therethrough which taper slightly at inlet portions in conical fashion in the flow direction, the portion of the metallic body located between the passageways having a volume equal to or greater than the volume of all of the passageways, and the metallic body being heated by disc-like ceramic PTC resistors which are attached to a surface of the metallic body by a thermally and electrically conductive plastic adhesive.
2. Discussion of the Related Art
It is known to use a ceramic PTC resistor, also referred to as a positive temperature coefficient or PTC resistor, as a heater or heating element. A ceramic PTC resistor (i.e., PTC resistor) consists of doped polycrystalline ceramic material of a perovskite structure on a base of barium titanate, and has a fundamental characteristic of a combination of semiconduction and ferro-electricity. As a result of this characteristic, there exists a marked positive temperature coefficient for the ceramic PTC resistor within a specified temperature range, the so-called PTC effect. At a specified temperature, the Curie temperature, which is determined by the material composition of the ceramic PTC resistor, a sudden increase in the resistance of the ceramic PTC resistor occurs, and this increase is by a few powers of ten.
If the ceramic PTC resistor is traversed by a current, it is heated until it reaches the Curie temperature at which, as a result of the sudden increase in resistance, it is scarcely traversed by the current, the energy consumption of the ceramic PTC resistor stabilizes and the ceramic cold conductor begins to cool. As soon as the ceramic PTC resistor has cooled it can again be traversed by current and again become heated. This heat up and cool down process continues in a cyclical fashion to provide a self regulating function of heating of the ceramic PTC resistor. Overheating and, as a result, destruction of the ceramic PTC resistor is thereby prevented. Thus, a ceramic PTC resistor is particularly suited for use as a heating element because of its self regulating characteristic.
The maximum temperature of a heating element made of a ceramic PTC resistor can be adjusted by changing its material composition. At present, maximum temperatures of up to 320.degree. C. can be achieved.
As a rule, ceramic PTC resistors are produced in the form of discs or thin plates, to two oppositely located, large surfaces of which are applied blocking-layer-free metal electrode securing a low contact resistance which contain, for example, silver or nickel. It is known that the ceramic PTC resistor material has a marked sensitivity to specific external influences in the surface area in contact with the metal electrodes as the PTC effect is operative only when a purposive metal covering securing a low contact resistance is provided. Therefore, the metal covering is provided between the ceramic PTC resistor material and the metallic electrode to prevent development of a blocking layer. The metal covering must, in the same way as the ceramic PTC resistor itself, be protected from harmful influences.
The use of ceramic PTC resistors to heat flowing media is known. U.S. Pat. No. 4,334,141, which claims priority from the application resulting in German patent No. PS 28 04 818, discloses an electrical heating device for beverage preparation machines, and the heating action is based upon the use of PTC heating elements. The heating elements are insulated by layers of electrically insulating but good heat conducting material to form heating plate segments. Free spaces between adjacent heating plate segments can be filled with an electrically insulating but good heat conducting filler compound.
German patent application No. OS 28 04 749 and German patent No. 28 04 749 disclose a continuous heater having heating elements which consist of PTC effect ceramic resistor and heat exchangers which consist of cylindrical sectors. The heating elements and heat exchangers form a fundamentally cylindrical structure. The cylindrical sectors are peripherally attached to one another by a cylindrical casing and the heating elements are located between surfaces of adjacent cylindrical sectors. The heating elements are pressed by the surfaces of the sectors when pressure is exerted between the cylindrical sectors.
In order to achieve an electrically insulating but good heat conducting connection between the heating elements and the cylindrical sectors, aluminum oxide ceramic is placed between each heating element and each cylindrical sector. Any spaces not filled by the aluminum oxide ceramic are cast with a heat conducting and electrically insulating filler compound such as silicone rubber.
German patent application No. OS 31 19 302 discloses an air heater. Metal heat-irradiating arrangements are in contact with surfaces of electrodes of respective heating elements which have a positive temperature coefficient. The heating elements can be clamped between two irradiating arrangements by means of heat resistant and electrically conducting silicone adhesive layers. Projecting parts of the electrodes of the heating elements can be connected by conducting wires to electrically conductive adhesive layers. However, the conduction wires can also be connected directly to the irradiating arrangements by means of a heat resistant and electrically conductive adhesive.
Electrically and thermally conductive adhesives for high temperatures are disclosed in U.S. Pat. No. 3,898,422. However, in this patent, a PTC heating element is attached only on one side by means of one of such adhesives to an object which is to be heated. A second side of the PTC heating element is contacted by a clamping spring.
U.S. Pat. No. 4,346,285 discloses another heater which uses a PTC element. Heat irradiating bodies which consist of a good heat conducting material contain holes through which a medium which is to be heated flows. The heat irradiating bodies are connected in heat conducting fashion to the PTC element by the clamping effect of a screw connection or via an electrically insulating adhesive. When an electrically conductive adhesive is used, the heat irradiating bodies are electrically insulated from the PTC element via an additional intermediate layer. In order to protect the PTC element from the medium to be heated, the PTC elements can be surrounded by a ring of plastic material. In order to not disturb the clamping effect of a screw connection, the thickness of this ring is slightly less than the thickness of the PTC element. However, this solution does not provide complete protection because, due to the thickness tolerances of the ring and the PTC element, complete sealing is not ensured.
European Patent Office patent application No. 0 194 507 discloses a heating element for heating flowing media in which a heat exchanger consists of a metallic body which is heated by disc-like ceramic PTC resistors which are attached to a part of the surface of the metallic body by a plastic adhesive. The metallic body consists of good heat conducting metal and is provided with regularly arranged passageways, the proportion of the total volume of the passageways to the overall volume of the metallic body being less than 50%. The ceramic PTC resistors are bonded by adhesive to oppositely located parts of the outer surface of the metallic body, possibly in recesses contained within the metallic body. Thus, in the disclosed heating element, only a one-sided coupling of the heat output of the ceramic PTC resistor is employed. This use of one-sided output coupling reduces the efficiency of the heating element.